virtio-gpu: add plane size
virtio-gpu: add plane size

This is useful if the guests wants to map a host GBM
buffer.

BUG=chromium:924405
TEST=compile

Change-Id: I7fb68aa49c4ce4f2a020b8ce96ad558290380b5f
Reviewed-on: https://chromium-review.googlesource.com/1591460
Commit-Ready: ChromeOS CL Exonerator Bot <fa429e5907f00867f797423aea74327bb02c91b6@appspot.gserviceaccount.com>
Tested-by: Gurchetan Singh <99434bba058e25b6203b8ae055a4f697dad0ef64@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Legacy-Commit-Queue: Commit Bot <5c2bf027dc5cc7ef186314391646e49f6866dc00@chromium.org>
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm